Revert "Lock toolbars while popups pointing to them are open"

This reverts commit 795c9ed2ed446fe52b56308a087e0a6a9ac2d8b3.  It was
causing the contents of the page to jump up when the toolbar was locked.
Open #46.
Hold toolbar open by adjusting timeout

The MainView adjusts the layout when the toolbar is locked open, which
we don't want.  So instead, we turn off the timeout when the popover is
open, so the toolbar will stay open until the popover is closed.  In
anticipation of the new header, we do this only on the book page.
Closes #46.
Set window title when reading book

There doesn't seem to be a good way to override the window title all the time, but for a single page that will only be at the top of the PageStack, it's possible.  The window title is most annoying for the BookPage, so set it for that one.  Ref. #48.
Use the new header, except on the BookPage

Refs #49.
Move "Reload" into settings dialog

This avoids the drop-down menu for actions on the Library page.  Refs #49.
Remove redundant title
Move the "Get Books" page into the PageStack

Instead of having a drop down menu with one other tab, just make an
action that moves to that page.  The "Get Books" page was never really a
peer of the "Library" page, anyway.  Refs #49.
Make fake target for sort popover

The ToolButtons in the new header don't have the positioning data that
popovers need to position themselves correctly.  To work around this
problem, we place an invisible item in the location of the ToolButton
and point the popover to that.  Refs #49.
Enable Actions, not ToolbarButtons

This is necessary for proper display in the new headers, so we only
really need to do this for the BrowserPage.  But we do it everywhere for
consistency.  Closes #49.